CPT Violation: What and where to look for

In this review I classify the possible ways of CPT violation, and I describe briefly their phenomenology, in both terrestrial and astrophysical experiments, including antimatter factories, neutral mesons and neutrinos, and discuss the various sensitivities. I also pay attention to disentangling genuine quantum-gravity induced CPT violation from `fake' violation due to ordinary matter effects. A particularly interesting situation arises when the breaking of CPT invariance is through unitarity violations, in the sense of the matter theory being viewed as an effective field theory, entangled with decoherening quantum gravity ``environments''. In such a case the quantum mechanical CPT operator is ill defined due to another mathematical theorem, and one has novel effects associated with CPT Violating modifications of Einstein-Podolsky-Rosen type correlations of entangled meson states in B and phi meson factories.